The UK’s solar panels generated more electricity than coal across the past six months combined, Carbon Brief analysis shows, rounding off a historic half-year of firsts.

Saturday 9 April 2016 was the first-ever day where more electricity was generated in the UK by solar than by coal. May 2016 was the first-ever month. The three months from June through to September was the first-ever quarter. And now the six months to September is the first half year.

These firsts reflect the changing face of UK electricity supplies, with solar capacity having nearly doubled during 2015. They also reflect historic lows for coal-fired generation, driven by changes in wholesale energy markets and the carbon price floor. Carbon Brief runs through the numbers.

Solar six months

The UK’s solar panels generated an estimated 6,964 gigawatt hours (GWh) of electricity during quarter two (Q2) and three (Q3) of 2016, from April through to September. (See note below regarding data sources and methodology).

The solar output was equivalent to 5.2% of UK electricity demand for the half-year period. It was nearly 10% higher than the 6,342 GWh generated by coal, which covered 4.7% of demand.

Starting on 1 July, there were 10 straight weeks when solar output exceeded that from coal.

Solar output is strongly affected by the UK’s seasonal cycle. Roughly three-quarters of annual UK solar power is generated during the sunnier half-year from April to September. In contrast, coal generation tends to increase in winter when electricity demand peaks.

The chart below shows these contrasting seasonal cycles. It also shows two contrasting broader trends.

First, UK solar capacity has to date reached around 12 gigawatts (GW), according to research by Solar Intelligence, up from around 6GW at the start of 2015. Solar generation is increasing as a result, up 26% in 2016 to date, compared to the same period in 2015.

(Note that solar capacity additions have fallen this year, following subsidy cuts. Note also that while government figures for new capacity have been consistently too low, independent estimates also show the drop.)

Second, the chart shows how coal generation has fallen rapidly, at a rate that is far beyond its usual annual cycle. Output in 2016 to date was 65% below that in 2015. It was down 76% in Q2 and 82% in Q3 compared to a year earlier.

This year also saw UK coal generation fall to zero on 9 April, for the first time since 1882, when a coal-fired power station started supplying electricity to the public for the first time. Since then, there have been 199 hours when coal was generating no power in the UK.

The drop in coal output has come about because of wholesale energy market price shifts being more favourable to gas-fired generators than to coal. In addition, the UK’s carbon floor price doubled in April 2015, again shifting the economics of electricity generation in favour of gas over coal.

Given these price changes, and the government’s stated intention to phase out all unabated coal by 2025, three coal-fired power stations took the decision to close this spring.

The key role of the carbon floor price in driving coal off the system is underlined in recent analysis from consultancy Cornwall Energy. This shows that removing the UK’s top-up carbon tax would mean coal plants once again being cheaper to run than gas.

“This would return the market to the position seen in 2014 when coal-fired generators were running baseload [all the time] and gas-fired stations were pushed to the margin.”

It’s worth noting that while gas-fired power stations have replaced most of the reduction in coal output, the total supplied by the two fossil fuels is also falling. This is because of increases in electricity supplied by renewables and imports, along with falling demand.

Methods: The figures for shares of total UK electricity generation are estimates. They only include solar generation and other forms of generation that are connected to the transmission grid network. Embedded generation from wind or other sources is not centrally metered and data is not available. However, this missing data will not alter the relative positions of solar and coal generation.

Figures for solar output in the UK are estimates produced by Sheffield Solar. The project recently updated its estimates of installed UK solar capacity. Its estimates scale real live data obtained from 324 solar sites around the UK.

Carbon Brief analysis shows the Sheffield Solar estimates to have a very small average error of 4%, compared to official government figures for solar generation since the start of 2015.